Cracking furnaces long have been used in the process of cracking a variety of hydrocarbon feedstocks to ethylene and other valuable olefinic gases. For the past 20 or 30 years, pyrolysis cracking at relatively short residence times and relatively high temperatures in furnace reactors has been the favored process for the production of ethylene, which is used as a precursor for several kinds of plastics (e.g., polyethylene, polystyrene, and PVC) and other everyday items such as ethylene-glycol (antifreeze). U.S. Pat. Nos. 2,671,198; 3,407,789; 3,671,198; 4,342,642; 4,499,055 and 5,427,655 illustrate basic designs of such short-residence time/high temperature cracking furnaces.
When thermally cracking saturated hydrocarbons to olefinic hydrocarbons—such as the cracking of ethane to predominantly ethylene, or the cracking of heavier saturated hydrocarbons like those comprising a naphtha or gas oil feedstock to produce less saturated products, such as ethylene and other higher olefins—it is generally desirable to input that quantity of heat needed to effect cracking of the hydrocarbon feed very rapidly while reducing the time that the initial cracking product is exposed to the cracking heat in order to increase the selectivity of such cracking conversion. It is this concept that underlies the millisecond residence time at a high temperature that is now the preferred process for furnace cracking of hydrocarbon feeds.
The cracking furnace used in the cracking process is any directly fired device in which a hydrocarbon feed, in the presence of dilution steam, passes through reactor furnace tubes in which endothermic reactions take place to form a wide range of cracked products, including ethylene. A conventional cracking furnace generally comprises a refractory lined firebox containing a multiplicity of high alloy metal reactor furnace tubes through the interior of which flows the hydrocarbon feedstock to be cracked, together with a suitable amount of dilution steam. The sensible heat and the heat of cracking are supplied primarily by radiant heat from burners located on the floor and/or walls of the firebox. This heat transfers through the metallic reaction lines (reactor furnace tubes) into hydrocarbon feedstock that flows there within. A reaction line may be as long as 400 feet and/or coiled in a serpentine shape that runs vertically up and down in the firebox, or it may be as short as about 50 feet in a straight single pass through the firebox, with or without a ‘crank’ (see DiNicolantonio et al., U.S. Pat. No. 4,499,055; Wallace, U.S. Pat. No. 3,671,198; Parizot et al., U.S. Pat. No. 4,412,975). Intermediate lengths and other geometrical configurations also are possible and, indeed, currently practiced.
Cracking furnaces, as constructed today, provide for millisecond residence time at a maximum bulk fluid temperature of about 1625° F., and are, with respect to their radiant heated reactor furnace tubes, constructed of metallic materials. The fireboxes themselves, which may be lined with refractory materials, are capable of delivering a greater heat load than the metallic materials of the reactor furnace tubes can withstand. This maximum service temperature of the metallic materials, of which the reactor furnace tubes are constructed, limits the performance of the aforesaid reactor furnace tubes with regard to their capacity (which should be as high as possible), and their residence time (which should be as short as possible), and hence selectivity (to achieve the highest possible yield of valuable olefinic species like ethylene and propylene, for example).
To date, given the relatively high temperatures to which the reactor furnace tubes are exposed in a thermal cracking process, metallic materials have been preferred as the only materials for construction of such tubes. As reactor designers have strived for the higher capacity and higher selectivity in the process, which would result from the use of materials with higher maximum service temperature limits, they have steadily improved the properties of the metallic alloys from which the reactor furnace tubes are manufactured. In recent times, conventional reactor furnace tubes have been constructed of nickel-containing alloys. In general, the development of the nickel-containing alloys for reactor furnace tubes, in order to increase the maximum service temperature of the aforesaid reactor furnace tubes, has been accomplished by the addition of ever-increasing amounts of nickel. See, for example, Kleeman, U.S. Pat. No. 6,409,847. The best nickel-based alloys, however, still have maximum service temperatures of only around 2100° F. The exposure of conventional reactor furnace tubes to additional high temperatures will exacerbate the problems already existent with conventional reactor furnace tubes, which include, but are not limited to, accelerated coke formation, consequential carburization and creep elongation.
At high cracking temperatures, the nickel in conventional reactor furnace tubes acts as a catalyst for coke formation inside the line—a particular form of coke that is termed “catalytic coke.” Coke also forms on the walls of the metal lines as the result of the pyrolysis itself, i.e., the action of time and temperature (particularly the very hot wall temperature) on the coke precursor material produced in the reactant mass. This type of coke, having both a different formation mechanism and a different structure from catalytic coke, is known as “pyrolytic coke.” The coke formed by pyrolysis overlays on top of the catalytic coke in the reactor furnace tube. The pyrolytic coke, being a function of time, temperature and coke precursor material, increases in amount along the line length, peaking at the output end of the reaction line where time, temperature and precursors are at increased levels. For a recent example of a general discussion of cokeformation in the cracking field, see, for example, the following: Kinetic Modeling of Coke Formation during Steam Cracking, S. Wauters and G. B. Marin, Industrial & Engineering Chemistry Research, 41 (10), 2379-91; Comments on “Kinetic Modeling of Coke Formation during Steam Cracking,” Lyle F. Albright, Industrial & Engineering Chemistry Research, 41 (24), 6210-12; and Reply to Comments on “Kinetic Modeling of Coke Formation during Steam Cracking,” Marie-Françoise S. G. Reyniers, Sandra Wauters, and Guy B. Marin, Industrial & Engineering Chemistry Research, 41 (24), 6213-14.
Coke formation is deleterious to the process for a number of reasons. The deposition of coke on the insides of the reactor furnace tubes constricts the flow path for the hydrocarbons, causing an increased system pressure drop. The higher average hydrocarbon partial pressure reduces the selectivity of the process; and in extreme cases, the coke can cause maldistribution of flow (between parallel reactor furnace tubes) and, ultimately, a decrease in the furnace capacity. Additionally, the coke lay-down on the inside of the furnace tubes increases the resistance to heat transfer between the outside of the reactor tube wall and the bulk fluid flowing within the reactor tube. Consequently, the outside flue gas temperature, the firing rate and the outside tube wall temperature have to be increased in order to maintain the same temperature and/or conversion of the hydrocarbon fluid flowing within the tube. Eventually the outside temperature of the wall of the reactor tube can reach the maximum service limit for the material from which the tube is manufactured, under which circumstances the coke has to be removed by passing a mixture of steam and air through the tubes in order to convert the coke (basically carbon) to a mixture of carbon oxides. This process is known as “decoking.” Decoking consumes valuable resources and, in the case of conventional nickel-based metallic alloy reactor furnace tubes, reduces the life of the tubes. Tube life is reduced by a variety of mechanisms including, but not limited to, abrasion, thermal fatigue, and damage to the internal oxide protective layer.
By way of example, in the process of ethane cracking, generally the coke precursor material with the highest rate of coking from pyrolysis is acetylene, although species such ethylene, butadiene, and benzene also contribute to the coking. Coke produced by catalyzed reaction on the nickel in the tubes, on the other hand, can be formed from almost any hydrocarbon and, of course, at lower temperature levels and in less time than needed for pyrolytic coke.
In order to reduce catalytic coking in alloy reaction lines, those skilled in the art have employed a variety of means. For instance, sulfur dosing and chemical treatments have been used to suppress catalyst sites by cladding and bonding. Other surface treatments have been employed, such as coatings and vapor deposition of ceramic based chemicals. Benum et al., U.S. Pat. No. 5,630,887, describes a method for treating for furnace tubes to reduce carburization or coking. Similarly, Wynns, U.S. Pat. No. 6,139,649, describes a method for coating high temperature nickel chromium alloy products such as furnace tubes to reduce coking. Additionally, Mendez Acevedo et al., U.S. Pat. No. 6,475,647, describes a protective coating system for protecting stainless steel from coking and corrosion.
In addition, dilution steam is often used to reduce coke formation inside the lines and to lower hydrocarbon partial pressure to provide improved ethylene yields (selectivity). The use of dilution steam, however, also significantly increases the cost and complexity of operation by adding the need for steam-raising equipment and by reducing the reactor throughput resulting in expensive and cumbersome downstream operations to separate the water from the hydrocarbon products.
Despite such efforts, coking in the high temperature region of the reaction line continues, run lengths remain short and furnace shutdown is common.
Attempts to reduce coking by varying the materials used for reactor furnace tubes are found in the prior art. For example, the prior art describes the use of silicon ceramics for reactor furnace tube construction. For example, Winkler et al., U.S. Pat. No. 2,018,619, describes an apparatus for the pyrogenic conversion of hydrocarbons that uses reaction lines made from silicon powder; Endter et al., U.S. Pat. No. 2,987,382, describes a furnace for carrying out gas reactions in ceramic tubes; Coppola et al., U.S. Pat. No. 4,346,049, discloses silicon carbide powder compacts produced from alpha phase silicon carbide powder for forming furnace lines; and Williams et al., U.S. Pat. No. 5,254,318 describes lined tubes for high pressure reformer reactors. However, none of these references teach or suggest the improvements in the cracking process that are possible when using ceramic or ODS tubes at significantly higher temperatures than conventionally employed.
Additionally, European Patent Application EP 1 018 563 A1 discloses a heating furnace tube comprising a rare earth oxide particle dispersion iron alloy containing 17-26 wt. % of Cr and 2-6 wt. % of Al and a method for using and manufacturing such a heating line in locations where the coking and carburization problems occur during the process. Although, EP '563 briefly mentions that the entire tube can be constructed of the rare earth oxide particle dispersion iron alloy, at page 6, lines 42-45, the patentees specifically state that it is advisable to use the material only for tube portions where coking problems occur. Thus, EP '563 clearly teaches away from any benefit to the cracking process of constructing the entire tube of from the alloy. EP '563 also does not suggest in any way that the two characteristics of rare earth oxide particle dispersion iron alloy—being nickel-free and having a high maximum service temperature—can be used to produce a furnace reactor tube in which hydrocarbons can be processed and cracked at higher capacity, shorter residence time, higher selectivity and, under some circumstances, higher conversion than possible heretofore.
Finally, Tassen, C. S. and co-workers, in a paper entitled “High Temperature Service Experience and Corrosion Resistance for Mechanically Alloyed ODS Alloys,” Heat-Resistant Materials, Proceedings of the First International Conference, Fontana, Wis., 23-26 Sep., 1991, suggest that “. . . MA alloys should perform exceptionally well in . . . pyrolysis and steam methane reforming atmospheres . . . .” The assertion is, however, made entirely in the context of the superior carburization resistance of MA ODS (mechanical alloyed oxide dispersion strengthened) alloys. The paper does not teach or suggest in any way that ODS alloys reduce the formation of coke. Neither does the paper teach or suggest in any way that the two characteristics of rare earth oxide particle dispersion iron alloy—being nickel-free and having a high maximum service temperature—can be used to produce a furnace reactor tube in which hydrocarbons can be processed and cracked at higher capacity, shorter residence time, higher selectivity and, under some circumstances, higher conversion than possible heretofore.
Thus, while such prior art generally teaches the use of non-conventional reactor furnace tubes for reducing coking, the prior art does not teach or suggest that any benefit could be derived from a process for cracking a hydrocarbon feedstock into olefinic hydrocarbon products comprising cracking said hydrocarbons in a furnace at a temperature of above about 1300° F. in a reactor furnace tube assembly comprising at least one reactor furnace tube comprised of a temperature-resistant, non-nickel containing material.
Special mention is also made of Duncan et al., U.S. Pat. No. 6,383,455, and Duncan, U.S. Pat. No. 6,312,652, both of which disclose non-conventional reactors comprising ceramic components. Additionally, at the Eleventh Ethylene Forum on May 14-16, 1997 and at the 10th Ethylene Producers Conference in 1998, Messrs. Pham, Duncan and Gondolfe presented papers entitled “Emerging Technology: Ultra-High Conversion Steam Cracking for Ethylene Production Using Advanced Ceramics” and “Coke Free Cracking—Is It Possible,” respectively, which discussed investigations into the use of ceramics for ethylene furnaces, but did not disclose or teach that improvements in ethylene production were possible by operating a furnace at high temperatures with non-nickel containing high-temperature resistant reactor tube materials.